Chlorination of Side Chains: A Strategy for Achieving a High Open Circuit Voltage Over 1.0 V in Benzo[1,2-b:4,5-b′]dithiophene-Based Non-Fullerene Solar Cells

A benzo[1,2-b:4,5-b']dithiophene-based donor material with chlorine atoms substituted on its side chains, named PBClT, was designed and developed in order to enhance the open-circuit voltage (V-oc) without decreasing charge carrier transfer in the corresponding blend films. The chlorinated polymer, PBClT, was an excellent donor material possessing a blue-shifted absorbance, resulting in desired complementary light absorption with low-band gap acceptor materials, for example, ITIC. In addition, the multiple chlorination dramatically decreased the HOMO energy level of PBClT, and the V-oc of the corresponding device increased to 1.01 V, which is much higher than that of the nonchlorine analogous, PTB7-Th, with a V-oc of approximately 0.82 V. The GIWAXS experiments displayed that PBClT/ITIC blend films showed a "face-on" orientation, which suggested that the chlorine substituents on the side chains favored pi-pi stacking in the direction perpendicular to the electron flow in photovoltaic devices. Furthermore, the PBClT/ITIC blend film exhibited a pi-pi stacking distance of 3.85 angstrom, very close to the distance of its nonchlorine analogous blend film with a distance of approximately 3.74 angstrom, which resulted in a slightly decrease of current density after multiple-chlorine-atom substitution. Based on this result, the introduction of multiple chlorine atoms on the perpendicular side chains not only adjusted the molecular energy level of the polymer using the electron withdrawing ability of the chlorine atoms but also subtly avoided obvious morphological changes that could result from strong steric hindrance in the backbones of the polymer. The device based PBClT/ITIC achieved a maximum PCE of 8.46% with a high V-oc of 1.01 V, which is an improvement in the PCE of approximately 22% compared with the performance of PTB7-Th-based device in our parallel experiments.